Tuesday, March 06, 2007

Thoughts on how the Earth came to be capable of sustaining life.

This is about a bit of a theory that I have had for some time that puts together a lot of info from a lot of different topics that could explain the “Big Picture”. Why do we have a habitable planet when it appears that the odds, cosmologically speaking, are very long against it? When you step back and look at it in a dispassionate and objective way, it does appear that the Earth’s existence, in its present form, may represent a very unlikely occurrence.

I keep thinking that someone out there must have done this before. However, I have not quite seen it all assembled in this manner before.

Contrary to the process of normal scientific inquiry, I’ll start with my conclusion first. I’ll then explain how I got there.

Conclusion as Prologue

I don’t think that intelligent life is as prevalent out there in the universe that we might think it is. Certainly, given the immense size of the universe and the number of planets out there, something MUST have happened out there that resulted in a planet that resembles the Earth. But, I am thinking that the possibilities aren’t as high as some out there have proposed. Here is my reasoning.

A stable provider of light and energy

The first thing that we obviously need in order to concoct an environment friendly to life is a stable energy source. That is, of course, our Sun. Stars like ours are very common in the universe. However, so are many other kinds of stars. There are huge stars, such as Betelguese, which will live for many times the life of our Sun but whose surface temperature is little more than burning charcoal. There are tiny white dwarf stars which burn hot but have a relatively short lifetime. There are huge white hot stars, such as Rigel. There are all sorts of binary stars, which would make life difficult for planets to even exist, much less orbit the star in a stable orbit. There are variable stars, some of which oscillate in energy output by several orders of magnitude.

We must also have a stable local (galactically speaking) environment, free from harmful effects of such nasty, esoteric denizens of the universe, such as pulsars, quasars, x-ray bursters, etc.

To whit, the Earth does not exist by itself. The Earth exists within its own environment. In order for life to exist on this planet, we need a stable source of energy without any harmful interference from outside sources. With that accomplished, we can move on.

The habitable zone surrounding the Sun

The Earth is situated in an orbit around the Sun that lies within a relatively small window that would sustain life as we know it. Water is the key, of course. Without water, there would be no life as we know it. In our particular orbit, water is warm enough to remain liquid but yet, not hot enough to evaporate. Therefore, our Earth had to have formed within this small orbital window for to even allow a chance of life beginning. Also, the Earth had to be the right size and right density in order to maintain an thick (but not too think) atmosphere, once it had established one.

This habitable zone looks to extend somewhere between the orbit of Venus and the orbit of Mars. Any further closer to the Sun, and you have a baked ember of a planet like Mercury. Any further from the Sun, and you would have an iceball whose oceans are ice and whose atmosphere lies frozen on the ground. Fortunately, the Earth seems to reside in just the right place. We experience water in all three states (i.e., solid, liquid and gas). Life has been shown to be very resilient, as underwater hydrothermal vents teem with life and can withstand being frozen in the artic winters, only to be reborn during the spring and summer months. However, too much of either heat or cold would have doomed any aspiring life form on primordial Earth.

Where did the moon come from, and why is it important?

Why is the planet Earth the only one in the inner solar system that has a very large moon? Both Venus and Mars resemble the Earth, in a way. The size and distance from the Sun are all within some comparable parameters. Venus has no moon, and Mars has two small moons that probably don’t even deserve the really intriguing names with which they are endowed. Given those observations, one could conclude that something very unusual must have happened to the Earth back when the solar system was still a pretty dynamic place.

Most current theories regarding the formation of the Moon involve a collision between a proto-Earth and another Mars-sized planet or planetesimal. Computer simulations show that the only type of impact that would have resulted in enough material in an orbit that won’t fall back onto the Earth is a side-glancing blow. A direct hit would not have produced the “planetary pair” that we see today. The other theories, such as a planetary capture and both the Earth and Moon coalescing out of the same disk of material, don’t really stand up to computer simulations.

It appears to me that, although nothing is certain in the areas of scientific theory, that the collision theory is the one that stands up best when trying to explain the system that we have right now. And if the state of Uranus is any indication, collisions of major sized bodies early in the formation of the solar system was not an unlikely thing to occur. How else does one go about explaining that Uranus is tipped on its’ side, with it’s axis of rotation pointing in the plane of the solar system disk? Something pretty darn big must have hit that planet to make that happen.

The role that the Moon played, and continues to play, in creating an environment that can sustain life is discussed below.

Plate tectonics, volcanism and the atmosphere

What is another feature that the Earth has that neither Venus nor Mars has? The answer is tectonic plates. Unlike the Earth, neither of those two planets seem to have a crust made up of large pieces. This points to something unusual in the Earth’s past that did not occur with the other two. This is one connection that I have not seen anyone else make before. I believe that it is not a giant leap of deductive reasoning to think that the same planetary impact that ultimately resulted in the Moon also shattered the Earth to the point that the crust broke into continent-sized chunks that are still with us today.

A direct outcome of the fact that the Earth has huge plates that float upon the Earth’s mantel and grind into each other is earthquakes and volcanoes. Yes, Mars has evidence of volcanoes; really big ones, in fact. But there is no evidence that any of Mars’ volcanoes are still active, and it certainly doesn’t have anything resembling the Earth’s “Pacific Rim Ring of Fire”, which contains a vast number of volcanoes, both active and dormant.

Volcanoes provide an excellent method of establishing a planetary atmosphere. Certainly, it’s very toxic at first. But even a toxic atmosphere establishes several things. One, it provides a basis for a constant energy exchange between the oceans and the atmosphere. It evens out the temperature around the globe, such that if one portion of the atmosphere becomes very hot or very cold, the law of thermodynamics rather insists huge extremes do not exist. Water is transported around the globe to places that normally wouldn’t see any water. It’s acts as a protective covering for the water that does occupies a large portion of the surface of the globe, such that it doesn’t all just evaporate into space and lost forever.

Of course, one form of life’s toxicity may be a fundamental requirement to another. High amounts of hydrogen in the atmosphere released by the volcanic activity enabled microbes to develop that consumed hydrogen and produced, as its waste product, free oxygen, which is one of the basic needs of our current form of life

Earth’s stable rotation

Another thing that the moon provides the Earth is a stabilizing force that helps keep the Earth in very stable rotation around its’ axis. The Earth rotates about its axis like a huge gyroscope. A gyroscope exhibits a number of behaviors including precession and nutation. If the Earth exhibited such behavior, the global climate would experience wild fluctuations, so much so that life would have had a difficult time to establish a toe-hold. I doubt life, during its very tenuous beginnings, would have fared well if its local climate were sub-tropical one year and arctic-like the next. Oh, it certainly may have, but I would imagine life would have had a much harder time of it without a stable local environment.

However, the Moon provides a giant “stabilizing force”, which forces the Earth’s to have a very stable rotation. This allows a very predictable, long-term environment to develop which would be infinitely more conducive to allowing life to develop than the one described above.

Tides and tide pools

There are still varying theories about how life actually came be on planet Earth. Did a soupy combination of organic materials spring to life on this planet? Were, somehow, life containing molecules deposited on the Earth from cosmic bombardments? However it happened, it is likely that life really was able to itself the interface between water and land. Tide pools provide a shelter from the direct effects of the ocean’s waves, while allowing in replenished water and nutrients on a regular basis.

From solstation.com: Bacteria formed microbial mats on land as early as three billion years ago. Fossilized remnants and other biochemical evidence from South Africa suggest that photosynthetic bacteria (primarily blue-green cyanobacteria, that may have included the ancestors of Chroococcidiopsis) may have colonized the wet surface of clay-rich soil during rainy seasons, but were blanketed by aerosol deposits laid down during subsequent dry seasons. Such mats may have formed in surface pools, water edges, and other wet spots on land (Press briefs from the NASA Astrobiology Institute of 2/5/01 and 11/29/00, and from the University of Pennsylvania).The Moon is responsible for the regular tides in the ocean, which enables the life enabling tide pools to exist.

The magnetosphere

Another thing that the Earth has that Mars doesn’t, besides oceans, a thick atmosphere, plate tectonics, active volcanoes and a very large moon is a very large magnetic field surrounding the planet. In fact, of all the four inner, rocky planets in our solar system, the Earth is the only one with a large magnetic field. Given this rather statistically limited number of examples, it still must be assumed a magnetosphere is not a given for planets otherwise capable of harboring life (caveat: life “as we know it”).

Why is this important? Well, because our planet needs to be in rather close proximity to the Sun in order to receive the amount of energy necessary for life, we are then exposed to different type of hazard; the solar wind. The solar wind, which sounds harmless enough, is the violent stream of highly energized particles coming off of the sun’s corona. The radiation alone, if allowed to pass directly onto the Earth’s surface, would be enough to disrupt and even damage fragile organisms. As an illustration, all astronauts in the International Space Station are ordered into a very protected room where they cannot be harmed by the gamma rays that result whenever very large sunspot activity occurs.

However, with the Earth’s substantial magnetosphere as protection, we have an “umbrella” in place that protects the entire planet from these harmful rays. One need look no further than the Aurora Borealis (that is, the “Northern Lights”) to see this protective umbrella in place. All the highly charged particles are funneled north or south into the Earth’s magnetic poles. The interaction of these particles with themselves and with the Earth’s atmosphere can be seen as a brilliant curtain of light. While beautiful and awe-inspiring, the Aurora is rather a sideshow compared to the most beneficial aspect of the Earth’s magnetosphere. Without such a protective umbrella, the Earth may have lost all of its atmosphere, as well as all free water vapor (including the oceans) to the unrelenting blast from the solar wind over the course of millions of years. The Earth would look like a bigger version of Mars; lifeless (as far as we know) and water only exists in protected locations such as the poles, in the perpetual shadows of crater walls, and underground.

If the Earth had no magnetic field, it is doubtful Earth would have been able to harbor such abundant life.

The giant gas planets as protectors

One other destabilizing force in the rise of life on Earth is having your global climate very suddenly changed due to impacts of huge meteors and comets. As most laypeople know, one of the leading theories about why dinosaurs are extinct is that the Earth was struck by a large comet, meteor, or asteroid, and this caused both short and long term effects that altered the dinosaur’s habitat so significantly that they were not able to continue as a species. This theory is still under debate, and I won’t go into that here. But I think it is safe to say that life would have had a very difficult time of it if faced with disruptions of this magnitude occurred with any sort of regularity. The amount of energy released during one of these collisions is enormous, as we witnessed in 1994 when multiple fragments of Comet Shumaker-Levy 9 slammed into the planet Jupiter. If the Earth were to sustain one of these types of impacts, as an wild guess, every 10,000 years, Earth’s entire ecosystem would essentially be “starting over” each time.

Therefore, it is very indeed fortunate that our solar system is equipped with a giant vacuum cleaner, in the form of four gas giants (Jupiter, Saturn, Uranus and Neptune) that operate in the outer portion of our system to sweep the solar area clear of most (but not all) dangerous rogue floating space bodies. Without these impassive hulking planets to act as our protector, Earth might still be experiencing a pounding that all planets and moons experienced during the formation of the solar system. One need look no further than the surface of the Moon or the moons of Jupiter and Saturn to see the violent history of planetary formation recorded in their scarred and broken surfaces.

Jupiter’s stable orbit.

It is also a very fortunate thing that these same gas giants are in a stable orbit around our Sun. This does not appear to be a given. There are something in the excess of one hundred extra-solar planets that have been discovered orbiting around other stars. Given the limitations of our detection capabilities, scientists and astronomers, so far, have been limited to detecting very large planets, on the order of Jupiter. However, what has startled astronomers has been the fact that many of these planets appear to be orbiting their parent star in a very close orbit. That is a mystery that has yet to be explained. No theory of planetary formation has yet come up with a explanation of how a large gas giant could have evolved in that close proximity to a star. The material which makes up the planet should have been subsumed by the star’s accretion disk during the star’s formation. So, how did those huge gas-giant planets find themselves in such a close orbit to their suns?

One theory that seems to explain this seeming mystery is as follows. If the planet could not have formed that close to its sun, then it didn’t. So, how did it get there, there? Applying either Occam’s Razor or Sherlock Holmes (whichever suits), the planet therefore must have moved there. Where did it come from? From a decaying orbit much further from the star, where it could have formed in the same way that Jupiter and Saturn formed in our solar system. However, for whatever reason (but that reason seems to be somewhat prevalent in the universe, based on the number of examples astronomers have discovered) the planet, over the eons, had a decaying orbit, which caused the planet to slowly spiral inward to where it is now in close proximity to its sun.

This goes without saying that it would be catastrophic to the rest of the planets in our solar system, if Jupiter or Saturn decided on following the same course. Any one of the four gas giants would gobble up any other planet, large or small, that lies between it and the sun. Therefore, it is indeed fortunate for us that all four outer gas giants stayed put in their own stable orbits after they formed. Otherwise, they would have been vacuuming up Earth, Mars and Venus instead of just cleaning up stray comets and asteroids.

Conclusions revisited

Given all these rather amazing circumstances surrounding the Earth and its neighborhood, I can only conclude that we are very fortunate to have such a pleasant and nurturing environment in which we live out our collective mortal existence. If you remove any single one of the circumstances described above from the history of our solar system, the Earth today would be a drastically different place than it is and may not have been capable of sustaining life. The Earth might resemble the hellish planet of Venus, or the dry, cold and airless planet Mars, instead of the relative Garden of Eden it does now, or may not even exist at all.

Of course, “scratching the surface” doesn’t even come close to describing our limited observations about the universe. There are millions of galaxies out there, each containing billions of stars. That’s a lot of opportunities to roll the equivalent of two thousand “snakes eyes” in a row while playing dice, which it looks comparable to what occurred in the history of our solar system. All the necessary and sometimes downright unusual events necessary to provide us with this environment conducive to life and protecting us from all the harsh conditions that exist in the universe occurred. How likely is that? Not very likely, in my mind. That is why I said, at the beginning, that intelligent life in the universe may not be as high as some have supposed.

Now, as to how these developments all lined up in just the right way, some people would use this as evidence of a Devine Influence. Nothing this complicated or unlikely could have ever come about on its own. That is a very similar argument many people make when discussing evolution vs. creationism. The odds look impossibly long for it to have happened (i.e, 2000 snake eyes in a row), so therefore, “someone” must have been behind it. “In the Beginning, God created the Heaven and the Earth.”

A competing concept is sometimes referred to as “the Theory of Serendipity”. In short, it goes something like this. “Because we are here, it must have happened that way.” To me, that sounds more plausible than hypothesizing a “Creator” who concocted the entire universe but yet, somehow, exists outside of this observable universe. Someone has to win the lottery eventually. Why couldn’t it have been us?

Either way it ultimately came down, what we have right now is a planet that is warm, friendly, protective and nurturing which provides us our many needs, both real and imagined. We should treat it much better than we are currently doing. It may not be very long before it comes back to haunt us in a very big and unpleasant way.